Recently, the liquid-crystal display (LCD) devices of the OCB mode are developed in addition to those of a TN (Twisted Nematic) mode (see JP-A-2001-281707, JP-A-2005-234376, for example).
When to display images on the liquid-crystal display device of the OCB mode, it is necessary to transform an arrangement state of an OCB liquid crystal molecules, from a splay alignment state to a bend alignment state as shown in FIG. 7, and to keep the bend alignment state for a while.
Reverse transition to the splay alignment state is prevented by applying a high voltage at a predetermined duration ratio in each of video frames. Visibility of the video image is known to be also improved when, as the high voltage for preventing the reverse transition, black-displaying voltage is applied as to achieve an insertion of black fractional frame in the each video frame.
When an image-displaying screen (viewing area) on display panel of the LCD device of the TN mode is viewed from a substantially perpendicular direction, a luminance decreases monotonously with increase in an applied voltage, and exhibits no particular voltage for minimizing the luminance; as no “minimal or local minimal value” is shown in a voltage-luminance curve in FIG. 8A for example. Therefore, no gray-scale inversion takes place and hence no significant deterioration of the visibility takes place so long as the display panel is viewed from a substantially perpendicular direction; even when the voltage-luminance curve varies or deviates within a viewing area on the display panel due to variation such as that of the thickness of the liquid crystal layer within the viewing area.
On contrary, a voltage-luminance curve for the OCB-mode LCD device, when an image-displaying screen is viewed perpendicularly from the front, assumes a substantially quadratic curve as shown in FIG. 8B for example. Thus, the “minimum or local minimum value” does appear in the voltage-luminance curve. The voltage-luminance curve or performance would vary or deviate within the viewing area; due to the variation of thickness of the liquid crystal layer (referred to “cell gap”) within the viewing area of the display panel. Thus, voltages in a range including right-hand and left-hand sides of a voltage for the “minimum or local minimum value” may be applied. Then, gray-scale inversion occurs even when the display screen is viewed perpendicularly, and hence the visibility is deteriorated.
It is aimed in view of the above, to curb gray-scale inversion that otherwise may occur even when the image-displaying screen is viewed perpendicularly, for the OCB-mode LCD device.